Laboratory instruments of a kind discussed herein serve, for example, as analytical balances in many fields of industry. Such fields of industry may include, without limitation, laboratories of research and development departments, as well as production areas (e.g., for quality control).
One exemplary analytical balance with a weighing compartment is described in detail in U.S. Pat. No. 4,700,793 A. Generally speaking, analytical balances are balances with a very high resolution of the measurement result. Consequently, even the smallest extraneous factors acting on the object being weighed or on the load receiver of the balance can introduce an error in the weighing result. The extraneous influence factors are rarely stable and this can lead to a situation where the precise weight of the weighing object cannot be determined. To protect the weighing system from being influenced by the environment, a weighing compartment is therefore commonly enclosed with a so-called draft shield.
As shown in U.S. Pat. No. 4,700,793 A, the typical draft shield of an analytical balance has in most cases two slidable sidewalls and sometimes also a slidable top cover, as the object to be weighted is normally delivered to the load receiver of the balance from the side, and sometimes also from above. A front wall of the draft shield is normally rigidly connected to the housing of the balance and, by functioning as a structural support, lends stability to the draft shield. The draft shield needs to be as tightly closed and solid as possible, so that air drafts of the ambient atmosphere cannot propagate into the weighing compartment through gaps and openings of the draft shield and cause atmospheric disturbances in the weighing compartment.
In order to make the weighing compartment, and in particular, the sidewalls of the draft shield easier to clean, a draft shield is proposed in U.S. Pat. No. 6,686,545 B2 whose front wall and sidewalls can be released from a form-fitting seat through a tilting movement and subsequently removed from the balance by a pulling movement. The top cover is connected through a linear guiding constraint to the balance housing, which serves as rear wall and can be slid horizontally over the balance housing, whereby the draft shield is opened at the top. In addition, the top cover, too, can be separated from the linear guiding constraint by means of a tilting movement.
It is further known in practice that the volume of the weighing compartment can limit the precision of a balance. The reason for this is that the enclosed air in a large weighing compartment is influenced much more strongly by the environment outside of the draft shield, for example by the incoming heat and light radiation. Further, in a large weighing compartment there are, in proportion to the enclosed volume, fewer surfaces slowing the air movement, for example after the sidewall has been closed. It therefore helps if the weighing compartment has a small volume in order for the air inside the weighing compartment to come to rest very quickly. Furthermore, the air in a weighing compartment of limited height is shifted about only to a minimal extent and it takes only a relatively short time for the air inside the weighing compartment to settle into a stable temperature profile over the height of the weighing compartment.
With the introduction of time-optimized work procedures, it occurs more often that different operations are performed at the same time or one after another directly in the weighing compartment without putting the object to be weighed on and off the load receiver several times. This is the case in particular in the preparation of solutions or powder mixtures.
The arrangement of a such multi-functional weighing compartment is disclosed for example in U.S. Pat. No. 6,603,081 B2. As shown therein, a multitude of accessories such as, for example, dosage-dispensing units, holder devices for source containers and receiving containers, protective covers, indicators and the like, can be installed temporarily in an arrangement that is optimized for a specific sequence of operations. Among the illustrated examples is an intermediate floor with a passage opening which can be fastened at an appropriate height dependent on the weighing container that is to be set on the balance pan. Thus, an analytical balance that was originally designed only for determining the mass of weighing objects is turned into a multi-functional laboratory instrument.
Although the intermediate floor which is disclosed in U.S. Pat. No. 6,603,081 B2 allows the volume of the weighing compartment to be partitioned and thus to be partially reduced, due to the existing draft shield the access to the opening in the intermediate floor and thus to the weighing compartment is strongly restricted by the protruding walls. Furthermore, it takes a certain amount of time to change the position of the intermediate floor. If the balance is used for example in a so-called glovebox, it can be extraordinarily difficult to raise or lower the intermediate floor.
However, the increase in functionality and the requirement for higher precision of the laboratory instruments of the foregoing description must not be allowed to restrict their user friendliness. The present invention is therefore directed to a draft shield for a laboratory instrument, which offers improved access to the weighing compartment while keeping the weighing compartment adaptable to the height of the weighing container.